1. Field
Aspects of the present invention relate to a lithium secondary battery and a method of controlling a short resistance thereof.
2. Description of the Related Technology
Due to the recent rapid development of compact and lightweight portable electronic devices, there is a growing demand for more compact and higher-capacity batteries as a driving power source thereof. Particularly, lithium secondary batteries have an operating voltage of 3.6 V or more, which is three times higher than the operating voltages of nickel-cadmium (Ni—Cd) batteries or nickel-metal hydride (Ni-MH) batteries, which are widely used as a power source of portable electronic devices. Further, lithium secondary batteries have a higher energy density per unit weight than Ni—Cd and Ni-MH batteries. For these reasons, the lithium secondary batteries have been rapidly developed.
A lithium secondary battery stores and releases electric energy by oxidation and reduction, when lithium ions are intercalated/deintercalated at a positive electrode and a negative electrode. A lithium secondary battery is manufactured using materials capable of reversibly intercalating and deintercalating lithium ions as active materials for the positive and negative electrodes, by charging an organic electrolyte or polymer electrolyte disposed between the positive electrode and the negative electrode.
A lithium secondary battery includes an electrode assembly, a can, and a cap assembly. The electrode assembly is formed in a jelly-roll shape, by winding a negative electrode, a positive electrode, and a separator disposed therebetween. The can houses the electrode assembly and an electrolyte. The cap assembly is assembled on the can.
Meanwhile, such a lithium secondary battery is charged or discharged by an electrochemical reaction occurring when ions are released, inserted, or moved between active materials of the electrodes. A repeatedly charged or discharged secondary battery may undergo an increase in internal pressure and heat, due to electrical misuse (overcharging) or other dangers. When such a state continues, the secondary battery may break or explode, thereby causing harm to a user. Thus, it is essential to prepare safety features to prevent this harm.
For example, a conventional secondary battery has a means for inhibiting a reaction, so that when an internal pressure is increased over a safe pressure, it blocks the conformation of an electric circuit, or breaks a safety vent in response to the pressure, thereby reducing the internal pressure and removing an electrolyte. An example of a conventional safety means is a porous separator installed between the positive electrode and the negative electrode. When a temperature in a case is increased over a safe temperature, the porous separator shuts pores down in response to the temperature, and inhibits movement of ions between the electrodes. In such a manner, the porous separator ensures safety, by inhibiting an electrochemical reaction (shut down).
However, when the temperature in the battery is excessively increased over a temperature release rate of the case, due to non-uniformity of the separator or other internal short circuits, the separator melts before the shut down occurs. As such, the separator is prevented from insulating the positive electrode from the negative electrode. In addition, when the positive electrode and the negative electrode are short-circuited, a chain reaction, including the decomposition of the negative electrode active material, the electrolyte, and the positive electrode active material (melt-down) occurs. As a result, a thermal runaway occurs, and the conventional secondary battery becomes unsafe and explodes.
Particularly, when a positive electrode collector and the negative electrode active material are short-circuited, such a melt-down phenomenon can bring a drastic increase in heating value, due to a resistance value at the short-circuited portion, and the occurrence of the thermal runaway. For this reason, an alternative for ensuring battery safety is needed.